Problems - instantiate, arrays

why cant I instantiate an object from an array like this, and how do I parent the current Instantiation to the previous instantiation

 var spawnpoints : GameObject[];
 
 var track : Transform[];
 
 var instant : GameObject[];
 
 var counter : boolean= false;
 
 var a : int;
 
 function Start () {
 instant[0] = Instantiate(track[Random.Range(0, track.length)], spawnpoints[Random.Range(0, spawnpoints.length)].transform.position);
 }
 
 function Update () {
 
  	for( a = 1; a<100; a++){
  		instant[a] = Instantiate(track[Random.Range(0, track.length)], spawnpoints[Random.Range(0, spawnpoints.length)].transform.position);
  		instant[a-1].transform.parent = instant[a].transfrom;
  	}
  }